Birth order linked to disease risk across millions of siblings in major study

A study of ten million siblings has surfaced a quiet but consequential pattern: the order in which a person is born into a family appears to shape their vulnerability to disease across a lifetime. Researchers mapped health outcomes against sibling sequence and found consistent, measurable differences between firstborns and later-borns — differences too large and too stable to dismiss. The finding invites medicine to reckon with a variable it has long treated as sociological rather than biological, and to ask what else it may have overlooked in the architecture of human health.